error in membership user creation

I have installed this plugin and i get this error after user creation:
Warning: Cannot modify header information - headers already sent by (output started at /nfs/c08/h01/mnt/115441/domains/...com/html/admin/wp-content/themes/fotografia/header.php:13) in /nfs/c08/h01/mnt/115441/domains/...com/html/admin/wp-includes/pluggable.php on line 680